1. n.A program used primarily to
create, manipulate, modify, or analyze other programs, such as a compiler
or an editor or a cross-referencing program. Oppose
app, operating system; see
also toolchain.

2. [Unix] An application program with a simple,
‘transparent’ (typically text-stream) interface designed
specifically to be used in programmed combination with other tools (see
filter, plumbing).

3. [MIT: general to students there] vi. To work; to study (connotes tedium). The
TMRC Dictionary defined this as “to set one's brain to the
grindstone”. See hack.

4. n. [MIT] A student who
studies too much and hacks too little. (MIT's student humor magazine
rejoices in the name Tool and Die.)